Bigtable: add a helper method to easily connect to an emulator

[igorbernstein2]

Extracted from #4498

[codecov]

Codecov Report

Merging #4506 into master will decrease coverage by <.01%.
The diff coverage is 0%.

@@             Coverage Diff              @@
##             master    #4506      +/-   ##
============================================
- Coverage     49.16%   49.15%   -0.01%     
  Complexity    21929    21929              
============================================
  Files          2077     2077              
  Lines        207074   207103      +29     
  Branches      24097    24097              
============================================
- Hits         101801   101795       -6     
- Misses        97100    97135      +35     
  Partials       8173     8173

Impacted Files	Coverage Δ	Complexity Δ
...e/cloud/bigtable/data/v2/BigtableDataSettings.java	24.52% <0%> (-10.61%)	3 <0> (ø)
.../bigtable/admin/v2/BigtableTableAdminSettings.java	69.04% <0%> (-30.96%)	6 <0> (ø)
.../java/com/google/cloud/testing/CommandWrapper.java	87.87% <0%> (-9.1%)	13% <0%> (ø)
.../cloud/datastore/testing/LocalDatastoreHelper.java	80% <0%> (-4.62%)	14% <0%> (ø)

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